This invention relates to a toner which is used to render visible an electric latent image or a magnetic latent image in electrophotographic processes, magnetic recording processes and the like, and more particularly to an encapsulated toner having an excellent pressure-fixable property.
Electrophotographic processes are image forming processes wherein a photoconductive phenomenon within solids is utilized to use a physical process or electron phenomenon. The electrophotographic processes are classified into xerographic, electrofax and persistent internal polarization modes and the like.
The xerographic process comprises the steps of applying an electrical charge to a photoconductive photoreceptor by means of a corona discharge or the like, then exposing the photoreceptor to light to form an electrostatic latent image, developing this latent image with fine grains, transferring the developed image to paper or some other material, and then fixing the transferred image.
While the principle of the electrofax process is the same as that of the xerographic process, it is characterized in that a photoconductive paper is used. The electrofax process comprises the steps of applying an electric charge to the surface of the photoconductive paper thereby to electrostatically charge the same, exposing it to light to form an electrostatic latent image, developing the latent image with fine grains, and then fixing it to form an image.
In any of the electrographic processes or the magnetic recording processes wherein the latent image is magnetically formed, a toner is used in rendering the latent image visible. A two-component development process wherein a mixture of toner and carrier particles is used and a single component development process wherein no carrier particles are used are known as the processes for rendering the latent image visible with the toner.
The image thus developed with the toner is transferred onto a recording support such as paper as needed, and thereafter a step for fixing a toner is provided in order to form a storable image. The fixing step is broadly classified into a heat-fixing process, a solvent-fixing process, and a pressure-fixing process.
In the heat fixing process, a toner image is melted and secured onto a recording support by means of heated rollers or a heating chamber to fix the same. Accordingly, it is necessary to provide a heat source. Further, in some cases it may be necessary to provide separately an apparatus for eliminating excess heat accumulated within the image forming apparatus. Furthermore, there are problems such as unavoidable enlargement of the image forming apparatus or increase of the amount of electric power used. In addition, in the heat fixing process, it is difficult to carry out high speed fixing because some time is required for melting the toner components.
In the solvent fixing process, a toner image is dissolved and secured onto a recording support by means of a solvent to fix the toner image. In this solvent fixing process, it is necessary that the solvent used be thoroughly removed from the fixed surfaced or support, and much labor is required to manage the toxicity, inflammability, odor, etc. of the solvent.
In order to solve the technical drawbacks accompanying the heat-fixing process or the solvent fixing process, a pressure-fixing process wherein a toner image formed on a recording support is fixed by pressure has been proposed and is attracting much attention. According to this pressure fixing process, the fixing of an image is carried out at the time when a recording support having a toner image formed thereon is passed between a pair of rollers. As a result, it is possible to carry out high speed fixing, and a heat source and electric power for heating are unnecessary.
However, according to the prior art pressure-fixing processes, a linear pressure as high as from 20 to 40 Kg/cm is generally required to fix a toner image, and therefore it is difficult to reduce the size and weight of the light image forming apparatus. Further, the recording support in some cases in the past has wrinkled or curled, and the strength of the paper has been reduced. Furthermore, since the image is formed by applying pressure to a particulate toner, the surface of the resulting image in some instances has disadvantageously been smoothed and become lustrous.
In order to solve the problems accompanying such a pressure-fixing process, the use of materials having a low softening point or melting point and having a low pressure-fixable property as a toner has been proposed. A toner comprising materials having a low softening point or melting point is accompanied by the following problems: interparticle agglomeration or blocking may occur during storage; the toner particles adhere to the interior of the image forming apparatus to contaminate the interior of the apparatus; and further when the toner fixed to the recording support is stored under some pressure, the toner adheres to the back of another support to cause damage or breakage of the image as well as the support.
Accordingly, a variety of encapsulated toners in each of which a low pressure-fixing material is used as a core and the outer surface of this material is coated with a resin which satisfies the characteristics required for toner powders to provide the core with an outer wall have been proposed.
The following toners have been proposed as these encapsulated toners.
(1) Double wall capsules obtained by providing the outer surface of a core material having a pressure-fixable property with a first wall comprising a hydrophilic film-forming polymeric material by a coacervation process or the like to form first capsule particles, dispersing the first capsule particles in an emulsion of various resins, and thereafter carrying out spray drying to provide the surface of the first capsule particles with a second wall comprising the resin described above, wherein colorants or magnetic particles are admixed into the core material or the outer wall described above (as disclosed in, for example, Japanese Patent Laid-Open Pub. No. 104829/1976, Japanese Patent Laid-Open Pub. No. 122449/1976, Japanese Patent Laid-Open Pub. No. 124934/1976, Japanese Patent Laid-Open Pub. No. 1028/1978, Japanese Patent Laid-Open Pub. No. 36243/1978, Japanese Patent Laid-Open Pub. No. 124435/1976, Japanese Patent Laid-Open Pub. No. 132838/1976, and Japanese Patent Laid-Open Pub. No. 113740/1977).
(2) Toners obtained by dispersing a core material having a pressure-fixable property as well as a constant particle size (from 3 to 25 microns) in a solution or emulsion of various resins and thereafter carrying out spray drying to provide the surface of the core material described above with an outer wall (as disclosed in, for example, Japanese Patent Laid-Open Pub. No. 18655/1980, Japanese Patent Laid-Open Pub. No. 89854/1980, Japanese Patent Laid-Open Pub. No. 166653/1980, Japanese Patent Laid-Open Pub. No. 89855/1980, and Japanese Patent Laid-Open Pub. No. 64349/1981).
(3) Toners obtained by providing the outside of a core material having a pressure-fixable property with an outer wall of resins such as melamine/formaldehyde resin, polyurethane resin or polyurea resin (as disclosed in, for example, Japanese Patent Laid-Open Pub. No. 142362/1980, Japanese Patent Laid-Open Pub. No. 21259/1983, Japanese Patent Laid-Open Pub. No. 66948/1983, Japanese Patent Laid-Open Pub. No. 66950/1983, Japanese Patent Laid-Open Pub. No. 68753/1983, and Japanese Patent Laid-Open Pub. No. 100855/1983).
However, the prior art encapsulated toners described above are accompanied by the following problems.
(a) In the case of the double wall capsule (1) described above, the first and second walls are provided on the core material in producing the toner, and therefore two steps are required. Accordingly, the production steps are complicated.
(b) In the case of the encapsulated capsule (2) described above, pulverization and classification steps are required in order to adjust the core material to a constant particle size (from 3 to 25 microns).
(c) In the case of the encapsulated toner (3) described above, it is necessary to use a solvent in which the core material is mixed, dispersed or dissolved, and in which one component of the wall materials is dissolved. This solvent remains in the encapsulated toner and adversely affects the toner characteristics. When no solvent is used, it is difficult to finely disperse the highly viscous mixture comprising the core material and one component of the wall material to the particle diameter required for the toner.
(d) Since the toners (1) and (2) described above are both dried by a spray drying process, these processes are advantageous in that the bulk of the dispersion medium can be removed in a short period of time. However, the agglomeration of particles due to their mutual collision in the spray drying step cannot be avoided, whereby it is difficult to obtain a toner having a particle size of from 5 to 30 microns in high yields.
Moreover, it is impossible to avoid perforation of the outer wall accompanying the rapid removal of the dispersion medium from the outer wall. If the outer wall of the toner becomes porous, the core material of the toner will leach out of the pores present in the outer wall, and consequently the toner particles will exhibit agglomeration or blocking to impair the toner fluidity. Thus, a porous outer wall is undesirable.
When an organic solvent is used as the dispersion medium or the like, the organic solvent evaporates off in the spray drying step. Therefore, the use of an organic solvent is undesirable because of hazards such as electrostatic explosion and toxicity. Further, the solvent must be thoroughly removed from the resulting toner.
(e) In any of the toners (1), (2) and (3) described above, it is necessary to control the particle size of the toner to be within an optimum range in the production steps. However, since the resin used for the outer wall is a cross-linking resin, inferior toners cannot be reused and thus pose economical problems.
(f) In the toners (1), (2) and (3) described above, when the amount of the toner electrically charged is adjusted, a charge control agent is confined within the particles of the encapsulated toner in any of the processes for producing the toners described above. However, the electrical charge characteristics of the toner particles primarily depend upon electrical characteristics of the outer surface of the toner particles. Accordingly, the processes for controlling the electrical charge according to the prior art are not necessarily effective processes. Therefore, in order to obtain ample charge-controlling characteristics, it is necessary to use a large amount of a relatively expensive charge control agent. Further, it is difficult to produce the toner in a manner such the charge characteristics of the toner will not deviate between production lots.